The present invention relates to a method and apparatus for measuring a telephone subscriber line exhibiting an impedance including resistors and capacitors connected in series-parallel form by means of a voltage controlled power source, a high-speed current measuring circuit and a signal processing technique from a telephone office in a short time with high accuracy.
In order to measure a capacitance of a telephone connected across a telephone line or a subscriber line, heretofore, the capacitance is charged sufficiently to a predetermined voltage through a high resistor and after the electric charges stored in the capacitance is discharged through the high resistor to a reference capacitor in a measuring device, a voltage stored in the reference capacitor is measured to calculate the capacitance of the telephone. The reason why the capacitor is charged and discharged through the high resistor is that the telephone is prevented from wrongly ringing by the charged and discharged current flowing through a subscriber's telephone in the measurement.
In order to prevent the wrong ringing of the telephone having a current sensitivity of 0.5 mA in a power supply of -48 V, it is necessary to charge and discharge the capacitance through a resistor of 100 k.OMEGA. or more. On the other hand, a time for charging and discharging the capacitance of the telephone of 3 .mu.F at the maximum through the resistor of 100 k.OMEGA. is considerably long and a charging or discharging time to 99% is 1.5 seconds.
Accordingly, a single measurement time is 3 seconds or more because of the charging and discharging time. Further, 9 seconds is required to measure three capacitances between the A line and the ground, the B line and the ground, and the A line and B line.
In a conventional measurement device, in order to measure an insulation resistance between the lines, a predetermined voltage is applied across the lines through a high reference resistor in the measurement device so that a line voltage at this time is measured to calculate the insulation resistance. In this case, it is required to measure the insulation resistance after the capacitance between the lines is charged sufficiently and the line voltage reaches a voltage obtained by dividing a voltage in the telephone office by the high reference resistor in the measurement device and the insulation resistance.
When the high reference resistor in the measurement device is 100 k.OMEGA. and the maximum capacitance across the lines is 3 .mu.F, the charging time to 99% is 1.5 seconds. Accordingly, a single measurement time is 1.5 seconds or more because of the charging time and three measurements including measurement of an insulation resistance between the A line and the ground, measurement of an insulation resistance and capacitance between the B line and the ground, and measurement an insulation resistance between the A and B lines require 4.5 seconds.
Further, in the conventional device, in order to exactly measure an external voltage on the lines, it is necessary to discharge electric charges across the lines through a high resistor. The reason why the electric charges are discharged through the high resistor is the same as in the above-mentioned measurement of the capacitance across the lines. In this case, a single measurement time is also 1.5 seconds or more because of the discharging time and two measurements including measurement of an external voltage and resistance between the A line and the ground and measurement of an external voltage between the B line and the ground require 3 seconds or more.
The foregoing is the reason of requiring a long time for the measurement due to the line capacitance, while each measurement of a voltage requires 0.5 second or more as a delay time of a low pass filter in order to remove a component of 10 V at the maximum of A.C. commercial frequency 50 Hz induced on the lines by means of the low pass filter.
The total time for all of the measurements is 20 seconds or more. Even if two measurement circuits are used to measure the A and B lines simultaneously, the total time for the measurements is 10 seconds or more, and the rationalization for maintenance and test is prevented.
The prior art technique is described in "Measuring Device for Test of Subscriber Line Using Electronic Circuit", the 1981 National Convention 408 of Information and System Part of the Institute of Electronics and Communication Engineers of Japan, and IEEE "Subscriber Line Testing for Digital Switching Office", Vol. 29, No. 10, pp. 1434-1441.
The prior art technique requires a long time for measurement of the telephone subscriber line and has a long charging and discharging time. Accordingly, remaining electric charges produce measurement error and measurement accuracy is also insufficient.